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I just stumbled across this website the other day, and this is my first time posting, so apologies if the question is too vague or improperly worded. I am trying to build an all ocean world. I'm curious how the ocean and wind currents would behave on a planet with no above-water land masses.

Here are a few relevant details:

  • The planet has about half earth's gravity.
  • The planet spins west to east, like the earth.
  • The planet has no axial tilt. However, it does have seasons, because of an elliptical orbit. At it's closest point to its star, the water is liquid and there is water evaporation on the equator, which leads to large clouds and rainstorms. At the planet's farthest point from its star, all water is frozen on the surface.

My assumption is that the ocean and wind would flow east to west, due to the planet's rotation being unbroken by any land. Some water would also flow towards the equator from both sides to fill the void left by evaporation. However, I am sure there are other factors I'm missing. Anymore thoughts would be most welcome! Thanks!

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  • $\begingroup$ Welcome to the site! I've made some edits to your question to add some clarity. If you disagree, feel free to roll them back to the original. $\endgroup$ – Frostfyre May 25 '15 at 2:35
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    $\begingroup$ The ellipsal revolution won't cause seasons. $\endgroup$ – Jimmy360 May 25 '15 at 3:14
  • $\begingroup$ @Jimmy360 Why not? Sunlight intensity varies as 1/r^2, so an orbit with an eccentricity of 0.5 (apoapsis is 3 times periapsis) gives 9 times as much sunlight during the closest approach as during the farthest point of the orbit. $\endgroup$ – AJMansfield Aug 4 '15 at 3:04
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Air currents are caused by the fact that as a planet spins on its axis, not all points on the surface move at the same angular velocity. Just as the center of a spinning record moves slower than the edge, the air above the equator moves faster than the air above regions more north or south. Because the ground itself is moving, the air currents themselves are more relative to each other than the ground, giving the effect of equatorial regions moving east to west, and non-equatorial regions moving west to east. This difference in air current causes vortexes, especially in the tropics, which are responsible for many weather patterns. This page describes the effect in more detail, including an image of the Earth's ocean air currents. Water currents also follow similar reasoning.

Further, equatorial regions are warmer, causing greater evaporation from the oceans and a drop in pressure. Water vapor, counter-intuitively, is less dense than air and therefore results in lower pressures than dry air. This contributes to the vortexes previously described, as dry air is pushed towards equatorial regions due to higher pressure.

Those basics addressed, on an aquatic world, air currents would be almost perfectly as I have described - west to east at the equator, east to west outside it, and spiraling in the tropics. If you look at the pacific in what I linked, you will see the sort of pattern you should expect on a planet with few land masses. However, since there would be nothing to determine where the vortexes start and end, as land masses generally determine that on Earth, tropical regions would be subject to sudden and intense air current (and thus weather) changes as these vortexes move around.

As a final note, axial tilt almost completely determines seasons. Elliptical orbits do not differ enough from longest to shortest to cause notable changes in temperature. The only way you could experience seasons from orbit would be if the orbit was extremely eccentric. I'm not versed well enough in astronomy to say much about that, but I suspect it would have negative consequences to life on the planet, such as possible disruption in the magnetic field.

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  • $\begingroup$ Thanks a lot for the detailed answer! The picture in the link was especially helpful. As far as seasons go, I was imagining an orbit that is eccentric enough to cause climate changes. I am still looking into that, but I read a couple articles that said NASA is looking at some planets with eccentric orbits as possible places with life, so I am assuming it is theoretically possible. Thanks again! $\endgroup$ – Cascadiahawk May 28 '15 at 4:46
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Weather:

Since your planet has some rather intense changes in temperature because of an elliptical orbit the weather would vary quite a bit during its trek. When the "warm pool" of water opens up on the equator warm air there would rise into vast storm complexes. As they continued to grow a lot of the cold air from the mid latitudes would rush downward into the warmer equatorial ocean to replace that rising air. This would form huge cyclonic storms reminiscent of Nor-Easters as they spiraled away from the equator. This would help to further break up the sea ice leading to still warmer waters, and a larger ocean surface as the process continues. More water would expand the storms from the equator outward as the ocean rapidly thawed. As the planet receded from the sun the storms would start to weaken as the oceans cooled. Ice would start to freeze over recently opened seas from the poles toward the equator. Eventually the entire ocean would freeze over again.

There is a problem with this planet though. Many studies suggest that an ocean world frozen completely like you mentioned would have a very difficult time thawing due to such a high and uniform albedo.

"If, for some reason, the pond froze over, it would likely never re-melt. Liquid water, being dark, absorbs most of the sunlight that hits it. But solid ice is very reflective. So if a planet’s oceans were to freeze, the amount of energy absorbed from the star would also drop, making the planet even colder. This feedback would push an icy eyeball into a completely ice-covered state from which it might never recover." http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets

If a frozen tidally locked world couldn't easily unfreeze, and it gets all the sunlight all the time, then how would your world unfreeze? Perhaps tidal stressing from a more massive neighbor like a gas giant parent planet? Or underwater geothermal activity that deposits dark material above the icy surface?

The main thing to think about is, it is probably take much more energy to thaw those oceans than you originally thought. And when that ice does thaw, the weather conditions will be that much more extreme. Unless you have a mechanism to open up the icy by other means, your going to need a lot of energy.

Ocean Currents:

As long as there is a temperature difference between two different sections of water, there will be currents. So during the summer months (or month) the currents would carry warmer water away from the equators toward the poles in long loops. This would also be drastically affected by the undersea topography. A shallow ocean with ridges might divert the current around or even concentrate it toward the surface. During winter time the currents would slow but still be present.

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The wind would flow from hot area to cold area. Since your planet is rotating, this makes the wind dynamic but predictable. In a short period of time, if you ere on a boat on the part of water that was closest to earth, all wind would flow away form you.

The wind and increased pressure would (generally) drive ocean currents away from the warm spot. So ocean currents would be dynamic but predictable.

Your planet would not have seasons... well it depends on how you define a season. If you gave your planet a slow rotation, a day could be like what we call a season. If, however, the rotation is like that of the earth, there will not be seasons.

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